High frequency signal transmission is most commonly point-to-point. Power is generated at a source (the transmitter), and delivered to a load (the receiver) via a transmission line. In such cases, the receiver usually includes a terminating resistance that is equal to the characteristic impedance of the transmission line. The transmitted signal power is dissipated in this resistance, and no signal reflection occurs from the receiver.
In some applications, it is desirable to transmit a signal from a single transmitter to multiple receivers arranged sequentially along the line. In such cases, each of the receivers necessarily receives only a fraction of the transmitted power, since it is shared among all of the receivers. Furthermore, the signal at each receiver arrives with a unique delay with respect to the transmitter, determined by the receiver's location. Impedance discontinuities in the signal transmission path are undesirable in this case, since they give rise to reflections that interfere with the original transmitted signal. In such arrangements, a simple terminating resistance alone is not sufficient to guarantee the integrity of the signal. In addition, a means is necessary to couple a fraction of the transmitted signal power to each of the receivers that are arranged along the line. A necessary requirement for these couplers is that they must not create local impedance discontinuities that would cause signal reflections.
A converse situation also occurs in which multiple transmitters are arranged serially along the transmission line, connected to a common receiver at the line's end. In this case, it is desired to aggregate the signal power of all of the transmitters at the receiver. Similar to the above case, the various signals each have a unique delay determined by the transmitter's location with respect to the receiver. Also as in the above case, signal reflections are a source of interference. The requirements for the structures that couple signal power between the transmitters and the transmission line are the same as in the above case.
More generally, distribution networks like the ones described above may be combined using active circuitry such that in some modes of operation the circuit at the end of the line transmits to multiple receivers arranged serially along the line, and in other modes the circuit at the end of the line receives signals from multiple transmitters arranged serially along the line.